The FIA has tonight issued the following:
"The FIA has asked Team Mercedes AMG Petronas F1 and Scuderia Ferrari Team which have taken part in tyre tests in the 2013 season to reply to a disciplinary inquiry in pursuance of the FIA Judicial and Disciplinary Rules.
This follows the Stewards' Report from the Monaco Grand Prix and represents supplementary information required by the FIA in the light of the replies received from Pirelli, who were asked for clarifications on Tuesday May 28th."
However, it should be noted that whilst Ferrari kept quiet about its test, which also took place at Barcelona but between the Bahrain and Spanish Grands Prix, it was with a 2011 car, unlike Mercedes which ran its current car and drivers.
Prior to Ferrari's test, Pirelli's testing was restricted to a 2010 Lotus run by the Enstone outfit's 'show car' team.
Run with the full knowledge of rival teams, it was made clear that full details of the tests would be shared, which they were, while Lotus would not gain an advantage.
Earlier today, Pirelli hosted a press conference at which it insisted that in addition to there being nothing secret about Mercedes test, and that it was purely focussed on 2014 tyres.
"The tests were performed with tyre specifications that are not used and will not be used in the 2013 world championship," said Paul Hembery. "The focus was on 2014."
Whilst insisting that there is no favouritism in terms of Mercedes or any other team, the manufacturer also stated that the prototype tyres to be used in Friday practice next week were not tested by Mercedes.